This project involves studies of the expression of intracisternal A-type particles and a nucleoplasmic protein (termed germinal vesicle antigen) in preimplantation mouse embryos. The mRNA species for these components will be detected and assayed by hybridization with complementary DNA probes produced to the related viral (particle) genomes occurring in tumor cells. Recombinant DNA clones containing the genome of intracisternal A-type particles will be selected from the mouse genomic library and used for studies of the genome organization of these particles. The major structural protein of A particles and the germinal vesicle antigen in embryos at different stages of development will be analyzed by immunological methods using specific antisera as well as by gel electrophoresis and cell fractionation.